Wheelchairs are land vehicles that are provided for transporting the physically impaired. Wheelchairs generally include a base frame supported on a supporting surface by a pair of idler wheels and a pair of drive wheels. The drive wheels may be manually driven by a wheelchair occupant or power driven by motors controlled by the wheelchair occupant. The wheelchair may be maneuvered by differentially driving the drive wheels. The idler wheels are generally in the form of casters, typically located in front of the drive wheels. The casters swivel to enhance the maneuverability of the wheelchair. Optionally, the wheelchair may include a pair of anti-tip wheels, typically located to rear of the drive wheels, which function to enhance the stability of the wheelchair.
The drive wheels have historically supported a rear portion of the wheelchair base frame while casters have supported a front portion of the wheelchair. However, the drive wheels of late have supported a front portion of the base frame as well. The former wheelchairs are commonly referred to as rear-wheel drive wheelchairs while the later wheelchairs are commonly referred to as mid-wheel drive wheelchairs. The casters of rear-wheel drive wheelchairs are typically located towards the front of the base frame. The casters of mid-wheel drive wheelchairs typically located towards the rear of the base frame.
Tipping is a potential problem with wheelchairs, and can occur when accelerating, or traversing obstacles or unlevel terrain. Tipping may also occur in wheelchairs that are provided with reclining seats. Anti-tip wheels are commonly attached to wheelchairs to reduce the risk of wheelchairs tipping over. Anti-tip wheels are usually attached to the rear end of rear-wheel drive wheelchairs and to the front end of mid-wheel drive wheelchairs. The anti-tip wheels are typically fixed in place or limited to a predetermined range of motion. This may have an undesirable affect on the operation of the wheelchair. For example, if the anti-tip wheels are fixed in place the mobility of the wheelchair may be impaired by the limited range of motion of the anti-tip wheels and make maneuverability difficult. Additionally, for example if the anti-tip wheels are limited to a predetermined range of motion and the range of motion is too large then the anti-tip wheels may fail to add stability since they would not provide resistance until after the wheelchair has become unstable. Conversely, if the range of motion is too small then the wheelchair may experience the same problems as when the anti-tip wheels are fixed in place.
It would be advantageous if there could be developed an improved anti-tip system that provides stability while maintaining the mobility and maneuverability of a wheelchair.